[clang][slh] Forward mSLH only to Clang CC1

Summary:
-mno-speculative-load-hardening isn't a cc1 option, therefore,
before this change:

clang -mno-speculative-load-hardening hello.cpp

would have the following error:

error: unknown argument: '-mno-speculative-load-hardening'

This change will only ever forward -mspeculative-load-hardening
which is a CC1 option based on which flag was passed to clang.

Also added a test that uses this option that fails if an error like the
above is ever thrown.

Thank you ericwf for help debugging and fixing this error.

Reviewers: chandlerc, EricWF

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54763

llvm-svn: 347582